There are many known call setup methods for use in setting up calls within the PSTN (public switched telephone network) though new methods are still evolving.
Data telephony, over a data network such as the Internet for example, will likely soon start to carry a significant amount of multimedia traffic, and has its own call setup methods for establishing calls over the relevant data network. Various different data networks exist including the Internet, wireline networks, cellular networks, and PCS (personal communications systems) networks for example. In addition, calls other than voice calls of course are establishable over such data networks. These include FAX calls, video calls, T.120 conference calls etc.
When a user wants to make a regular phone call, the user's phone informs a switch via a sequence of electrical signals, or a message. Software in the switch will engage a process to find out where the called phone is located by performing an address translation. The address translation will return the information related to which trunk and which protocol should be used to signal a remote CO (captive office) which will ring the called phone.
In the case of an H.323 terminal, a call setup request is sent in a message to a component called the gatekeeper to perform address translation. The address passed to the gatekeeper can be a symbolic name or an Internet address. In the current H.323 implementation, the gatekeeper will use the H.245 and H.225 protocols to complete the call.
It would be desirable to be able to place calls of various types from a PSTN based terminal for example for connection to a data network based terminal, and call setup methods must be established for this. Existing solutions have focused on a few particular instances of the problem such as Internet to PSTN voice calls.
These existing solutions provide a single solution for one or two media, but do not provide an approach to solving the problem in a general fashion.
One of the technical challenges in solving this problem is the complexity and diversity of the types of call processing equipment, call transmission media and protocols involved all of which are based on various technologies.